Methods and apparatus for a semiconductor device having bi-material die attach layer

ABSTRACT

Described examples include a device including a semiconductor die having a first surface with bond pads and an opposite second surface attached to a substrate by an adhesive layer covering at least a portion of the surface area of the second surface. The adhesive layer includes first zones composed of a first polymeric compound and adding up to a first portion of the surface area, and second zones composed of a second polymeric compound and adding up to a second portion of the surface area, the first zones and the second zones being contiguous. The first polymeric compound has a first modulus and the second polymeric compound has a second modulus greater than the first modulus.

TECHNICAL FIELD

This disclosure relates generally to the field of semiconductor devicesand processes, and more specifically to the structure and fabricationmethod of packaged semiconductor devices.

BACKGROUND

The dramatic increase of the world market price for gold compared toother metals stimulated the semiconductor device industry to developcopper wire ball bonding as a replacement of gold wire ball bonding.While copper is a welcome bond wire material because it has about 30%higher electrical conductivity when compared to gold, use of copper bondwires requires the formation of intermetallics with aluminum bond padsfor strong and reliable ball bonds. The need for the intermetallicformation necessitates the use of higher temperatures during the copperwire bonding process, when compared to the prior gold wire bondingprocesses. For copper wire ball bonding, temperatures above 220° C. andmore typically in the range of about 250° C. are needed, while for gold,temperatures in the range of 180° C. are sufficient. At these higherprocess temperatures, the elasticity modulus of the adhesive polymericmaterials used for the die attach layer between the back side of thesemiconductor die and a substrate decreases, causing a risk ofnon-stick-on-pad (NSOP) defects. This NSOP issue can be affected withlimited success by optimizing the curing profile of an adhesivepolymeric material. If the modulus of the adhesive polymeric material ishigh at high temperatures, then the material may be at risk ofdelamination during stress tests. Such stress tests involve, forinstance, repeated temperature cycles from −65° C. to +150° C., or 1000hr storage at 85° C. and 85% relative humidity and while applying anelectric bias to the packaged device.

While polymeric materials such as polyimides and epoxies are insulators,many integrated circuit device types require adhesive die attachmaterials that provide electrical and/or thermal conductivity.Consequently, many die attach materials, such as silver epoxy, include ametallic filler such as silver. In the electric field of a directcurrent (DC), for instance between an anodic die pad and a nearbycathodic bond pad, the silver filled epoxy can undergo dissolution inthe presence of moisture, creating positive silver ions. In the presenceof an adsorbed water layer acting as an electrolyte, the positive silverions can migrate in the DC electric field from the anodic die pad to thecathodic bond pad, where the ions are reduced to pure silver andeventually form a dendrite tree that can cause an electrical shortbetween anode and cathode. Improvements are therefore desirable.

SUMMARY

In a described example, a device includes a semiconductor die having afirst surface with bond pads and an opposite second surface attached toa substrate by an adhesive layer covering at least a portion of asurface area of the second surface. The adhesive layer includes firstzones composed of a first polymeric compound and adding up to a firstportion of the surface area, and second zones composed of a secondpolymeric compound and adding up to a second portion of the surfacearea, the first and second zones are contiguous. In one example, thefirst polymeric compound has a first modulus and the second polymericcompound has a second modulus greater than the first modulus. In anotherexample, the first polymeric compound includes metallic fillers, and thesecond polymeric compound is substantially free from the metallicfillers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cross section of an embodiment for a packagedintegrated circuit having a bi-material die attach layer connecting asemiconductor die to a leadframe.

FIG. 2 is a flow diagram summarizing an embodiment process flow forcreating a bi-material die attach layer.

FIG. 3A depicts a perspective view of a substrate shaped as a leadframe.

FIG. 3B shows a cross section of the leadframe of FIG. 3A.

FIG. 4A illustrates a perspective view of the process of forming anadditive layer of a first polymeric compound on the leadframe of FIG.3A.

FIG. 4B depicts a cross section of the additive layer of a firstpolymeric compound on the leadframe of FIG. 4A.

FIG. 5A shows a perspective view of the process of forming an additivelayer of a second polymeric compound contiguous with the layer of thefirst polymeric compound.

FIG. 5B illustrates a cross section of the additive layer of a secondpolymeric compound contiguous with the layer of the first polymericcompound.

FIG. 6A depicts a perspective view of the process of attaching aworkpiece shaped as a semiconductor die to the additive bi-materiallayer of the first and second polymeric compounds.

FIG. 6B shows a cross section of the semiconductor die attached to theadditive bi-material layer on the leadframe.

FIG. 7 illustrates a cross section of wire ball bonds interconnectingthe semiconductor die and the leads of the leadframe.

FIG. 8 shows a parallel lapping through the bi-material die attach layerof an embodiment such as the embodiment illustrated in FIG. 1.

FIG. 9 displays a parallel lapping through the bi-material die attachlayer of another embodiment.

FIG. 10 illustrates a parallel lapping through the bi-material dieattach layer of yet another embodiment.

FIG. 11 illustrates a parallel lapping through the bi-material dieattach layer of yet another embodiment.

FIG. 12 shows a parallel lapping through the bi-material die attachlayer of yet another embodiment.

FIG. 13 displays a parallel lapping through the bi-material die attachlayer of yet another embodiment.

FIG. 14 illustrates a parallel lapping through the bi-material dieattach layer of yet another embodiment.

DETAILED DESCRIPTION OF THE EXAMPLE EMBODIMENTS

Corresponding numerals and symbols in the different figures generallyrefer to corresponding parts unless otherwise indicated. The figures arenot necessarily drawn to scale. Following common practice, the parameterdescribing the elastic behavior of materials, the elasticity modulus, isoften referred to herein in abbreviated fashion as modulus.

In an embodiment of a semiconductor device with an integrated circuitdie, the adhesive layer for attaching the die is composed of distinctand contiguous zones made of different materials. As used herein, theterm “zone” refers to a fraction of the layer composed of only one typeof material. The first zones include a first polymeric compound with afirst modulus, and the second zones include a second polymeric compoundwith a second modulus greater than the first modulus. A layer of suchcomposition is referred to herein as a bi-material layer. Examples ofdescribed devices use a bi-material die attach layer as the adhesivelayer with a certain area to attach a semiconductor die to a substrate.The surface area of the die determines the area of the attach layer. Thearea of the die attach layer includes a first area portion composed ofthe sum of the first zones, and a second area portion composed of thesum of the second zones. In the embodiment, the second zones aredistributed to locations which correspond to (or mirror-image) thelocations of the die bond pads on the die top surface. Consequently, thesecond area portion, as the sum of the second zones, may be smaller thanthe first area portion as the sum of the first zones. In an embodiment,copper ball bonds are attached to the die bond pads; the copper wirebonding process uses temperatures of about 250° C. and appliesmechanical pressure onto the semiconductor die in order to attach theballs of wire bonds.

In another embodiment, a metallic filler is added to the first polymericcompound, while the second polymeric compound is essentially free ofmetallic fillers. The second zones of the second and filler-freecompound are distributed to locations where a metallic dendrite from theadhesive bi-material die attach layer to a bond pad could grow along thegradient of an electric field, if the metallic fillers (and moisture)were available. The use of these embodiments reduces the risk ofmetallic dendrite forming and possibly causing a short.

FIG. 1 illustrates an example embodiment with an adhesive layer, abi-material die attach layer, attaching a semiconductor die to asubstrate in a packaged integrated circuit device 100. In FIG. 1, thesemiconductor die 101 has a first surface 101 a, which includes aplurality of circuitry terminals configured as bond pads 102. Die 101 iscommonly silicon, but may be any other semiconductor material such asgallium nitride and can include epitaxial layers as the semiconductormaterial. Opposite first surface 101 a is second surface 101 b. The areaof second surface 101 b adheres to bi-material die attach layer 110having thickness 110 b and length 110 a. The bi-material die attachlayer 110 is adhesive. The embodiments are also applicable to exampleswhere the device is not a monolithic integrated circuit but theembodiment includes a device attached to a substrate.

The adhesive bi-material die attach layer 110, in turn, is adhering tosubstrate 120, which, in the case of device 100, is a metallic leadframewith die support pad 121 and leadframe leads 122. In other devices,substrate 120 may be a carrier laminated of several layers of metal andinsulators, or any organic substrate. For the device of FIG. 1,leadframe leads 122 are shaped as flat leads used in Quad Flat No-Lead(QFN) devices or in Small Outline No-Lead (SON) devices; in otherdevices that can be used with the embodiments, the leads may be shapedas cantilevered leads. The bond pads 102 of semiconductor die 100 have ametallurgy suitable for copper wire ball bonding; FIG. 1 shows copperballs 130 attached to bond pads 102 and copper wires 131 attached toleadframe leads 122 by stitch bonds. The device 100 is encapsulated in aplastic package preferably employing an encapsulation material 140 ofepoxy-based molding compound with inorganic fillers. Other moldingcompounds for integrated circuit packages can be used.

As FIG. 1 indicates, bi-material die attach layer 110 includes one ormore first zones 111 and one or more second zones 112; first zones 111and second zones 112 are contiguous. In the example of FIG. 1, firstzones 111 are composed of a first polymeric compound having a firstmodulus. The sum of the first zones 111 are in contact with surface 101b of semiconductor die 101. Second zones 112 are composed of a secondpolymeric compound having a second modulus greater than the firstmodulus. Due to the assembly process employed (see FIG. 6B), the secondpolymeric compound may form a meniscus 113 extending from die supportpad 121 to some height along the die sidewalls.

As a materials characteristic, the elasticity modulus E [measured inpascal, Pa], or in brief the modulus E [Pa], is related to stress σ [Pa]and strain ε by the relation of Equation 1:E [Pa]=σ [Pa]/ε  (1)

The modulus E is temperature dependent; it decreases with increasingtemperature. As an example, the modulus of polymeric materials used insemiconductor technology may drop an order of magnitude, or more, fromroom temperature to the higher temperatures needed for wire ballbonding.

In semiconductor technology, semiconductor dies are attached tosubstrates at room temperature, polymerized at elevated temperature (inan example process, a 30 min ramp from 25° C. to 175° C., hold at 175°C. for 1 hour, and then cool down to room temperature), and then broughtto the temperature of wire bonding. While the wire ball bondingtemperature is in the range of about 180° C. for traditional gold wires,for copper bond wires a temperature of >220° C., preferably in the rangeof 250° C., is increasingly used. Materials used as die attach compoundsfor attaching semiconductor dies to substrates are categorized as lowmodulus materials when E<100 MPa at the wire bonding temperature, and ashigh modulus materials when E>200 MPa at the wire bonding temperature.

For stress-sensitive semiconductor devices such as Bulk Acoustic Wave(BAW) devices and some Micro-Electro-Mechanical Systems (MEMS), the dieattach materials preferably exhibit low modulus values at the wire ballbond temperatures. Investigations by Applicants have demonstrated thatin the assembly of semiconductor devices, where the modulus E of the dieattach materials is low at the wire bonding temperature, semiconductordies have a tendency to show NSOP defect issues and can fail wirebonding.

In the embodiment of FIG. 1, the first modulus of the first polymericcompound for the first zones 111 belongs to the regime of low modulus(<100 MPa at wire bonding temperature), and the second modulus of thesecond polymeric compound for the second zones 112 belongs to the regimeof high modulus (>200 MPa at wire bonding temperature). As mentionedhereinabove, the first and the second compound are adhesive, and thefirst and second zones 111 and 112 are contiguous. The first and secondadhesive compounds are commercially available from the companies Henkel,Germany, and Sumitomo, Japan.

As mentioned, first zones 111 add up to a first portion of the surfacearea of second die surface 101 b, and second zones 112 add up to asecond portion of the surface area. In the example device of FIG. 1, thesecond portion is smaller than the first portion; in other deviceexamples, it may be the reverse. Furthermore, in the example of FIG. 1,the second zones 112 are distributed to locations of the bi-material dieattach layer 110 that correspond to locations of bond pads 102 on thefirst die surface 101 a. Consequently, the attachment process of thefree air balls during the wire bonding process with the necessarymechanical pressure and ultrasonic agitation of the bonder toolcapillary is supported by the stiffer characteristic of the secondpolymeric compound (see process flow described hereinbelow), while thecompliant characteristic of the first polymeric compound remainsavailable for protecting the stress-sensitive circuitry of die 101.

For devices which need enhanced electrical and thermal conductivity fromthe semiconductor die to the metallic substrate pad, it is advantageousto include metallic fillers (such as silver particles) in the firstpolymeric compound forming zones 111, while the second polymericcompound forming zones 112 (and meniscus 113) remains essentially freeof metallic fillers. This separation of the metallic filler from certainareas will prevent the formation of metallic dendrite trees (such assilver dendrites) as a result of metal migration along a die side walltowards the die top during biased highly accelerated stress tests(BHAST).

FIGS. 3A thru 7 illustrate steps of the fabrication process flow of anembodiment semiconductor device incorporating an adhesive die attachlayer including a first and a second polymeric compound. The processflow starts by providing a substrate for the device. When the substrateis a leadframe as illustrated in the example 320 of FIG. 3A with crosssection shown in FIG. 3B, such leadframe is preferably etched or stampedfrom a thin sheet of base metal such as copper, copper alloy,iron-nickel alloy, aluminum, Kovar™, and others, in a typical thicknessrange from 120 to 250 μm. As used herein, the term base metal has theconnotation of starting material and does not imply a chemicalcharacteristic. Some leadframes may have additional metal layers platedonto the complete or the partial surface areas of the base metal,examples are: nickel; palladium; and gold layers on leadframes.Materials used for coating leads for improving wire bond reliabilitysuch as electroless nickel/immersion gold (ENIG) and electroless nickel,electroless palladium, immersion gold (ENEPIG) can be used. Commerciallyavailable leadframes are sometimes referred to as “preplated” leadframeswhen these coatings are provided by a leadframe vendor. Alternativelythese coatings can be added by electroless or electroplating in a priorprocess step (not shown).

A leadframe provides a stable die support pad (321 in FIG. 3A) forfirmly positioning a semiconductor die. In the example leadframe of FIG.3A, die support pad 321 has rectangular shape. Further, a leadframeoffers a multitude of conductive leadframe leads 322 to bring variouselectrical conductors into close proximity of the die. Any remaining gapbetween the tip of the leads and the die terminals will typically bebridged by thin bonding wires (see FIG. 1); alternatively, in flip-chiptechnology the die terminals may be connected to the leads by metalbumps. For the leadframe, the desired shape of pad, leads, and othergeometrical features can be etched or stamped from the original metalsheet.

The leadframe characteristics should facilitate reliable adhesion toattached die and to packaging compounds. Besides chemical affinitybetween the molding compound and the metal finish of the leadframe,reliable adhesion may be increased by increasing leadframe surfaceroughness. This can be particularly helpful in view of the technicaltrend of shrinking package dimensions, which offers less surface areafor adhesion. The increasing use of lead-free solders pushes the solderreflow temperature range into the neighborhood of about 250° C., makingit more difficult to maintain mold compound adhesion to the leadframesat elevated temperatures. Adding roughness to the surface of theleadframe can further increase reliability, particularly when lead-freesolder and the corresponding increased solder reflow temperatures areused.

Referring now to the process flow of FIG. 2, during step 201 of theprocess flow and illustrated in FIG. 4A, equipment for dispensingpolymeric adhesive compounds is provided. In this example embodiment ofFIG. 4A, the equipment includes a computer-controlled inkjet printerwith movable first nozzle 401 and movable second nozzle 402. From thenozzles, discrete drops of the compounds can be dispensed. In oneexample, the first nozzle 401 dispenses a first polymeric adhesivecompound having a first modulus, and the second nozzle 402 dispenses asecond polymeric adhesive compound having a second modulus greater thanthe first modulus. In an alternative example, the first nozzle 401dispenses a first polymeric adhesive compound including metallicfillers, and the second nozzle 402 dispenses a second polymeric adhesivecompound that is free from fillers. The metallic fillers can be onesthat provide electrical conductivity, thermal conductivity, or canprovide both. Silver is an example filler material. Automated inkjetprinters can be selected from a number of commercially availableprinters including piezoelectric, thermal, acoustic, and electrostaticinkjet printers. Alternatively, a customized inkjet printer can bedesigned to work for specific polymeric compounds in the form of pastes.In alternative embodiments, the adhesives can be applied using otherequipment and methods, such as screen printing, flexographic printing,gravure printing, dip coating, and spray coating. Syringes can be used,or other dispensers can be used.

Referring to the process flow of FIG. 2, during step 202 and illustratedin FIGS. 4A and 5A the first nozzle 401 and the second nozzle 402 aremoved across the surface of substrate (leadframe) while dispensingalternatively, or simultaneously, first and second compounds foradditively depositing an adhesive die attach layer on the surface of diesupport pad 321. In another alternative approach, the substrates can bemoved relative to the nozzles while the compounds are dispensed from thenozzles. In this approach the nozzles can be fixed or can also move, thenozzles move relative to the surface of the substrate while dispensingthe compounds.

Referring to the process flow in FIG. 2, during step 203 and illustratedin FIG. 4A the first nozzle 401 dispenses ink droplets 401 a of thefirst polymeric compound and deposits first zones 411. The sum of firstzones 411 adds up to an attach layer covering a first portion of thesurface area corresponding to die surface 101 b in FIG. 1. FIG. 4A showsthat the first portion is shaped as a rectangle. The attach layer of thefirst portion has a low modulus at the temperature of wire ball bondingcompared to the high modulus of the second polymeric compound. FIG. 4Bdepicts a cross section of die support pad 321 with the deposited thefirst zones 411, which are shown to have a flat surface 411 a.

In further action during process step 203 and illustrated in FIG. 5A,the second nozzle 402 dispenses ink droplets 402 a of the secondpolymeric compound and deposits second zones 412. The sum of secondzones adds up to an attach layer covering a second portion of the areaconstituted by die surface 101 b (see FIG. 1) surface area, the firstand second zones being contiguous. FIG. 5A shows that the second portionis shaped as a frame surrounding the rectangle of first portion 411. Thecross section of FIG. 5B shows the complete deposited bi-material dieattach layer 510 and indicates that the surfaces 412 a of second zones412 are coplanar with the surface 411 a of the first zone 411. (FIGS. 8to 14 display a compilation of other configurations of bi-material dieattach layers where the first and second zones are arranged in variousalternative ways.)

During step 204 of the process flow of FIG. 2 and illustrated in FIGS.6A and 6B, the second surface 601 b (opposite the first surface 601 awith bonding pads 602) of a semiconductor die 601 is attached onto thedeposited adhesive layer 510. During the die attaching process, slightmechanical pressure exerted on the die squeezes bi-material die attachlayer 510 so that a small amount of the second polymeric compound insecond zone 412 is squeezed outward, forming a meniscus 613 stretchingfrom die support pad 321 upward to some height along the die sidewalls.Meniscus 613 is observable by visual inspection and can therefore beused as a means of process control. FIG. 6B depicts the correspondingand matching positions of bonding pads 602 and second zones 412 made ofa high modulus polymeric compound. FIG. 6B further illustrates thatfirst zones 411 made of a low modulus polymeric compound correspond thepositions of die circuitry, providing protection for stress sensitivecircuits (for example, BAW devices) and other stress sensitivecomponents such as MEMS.

During step 205 of the process flow of FIG. 2, the low modulus and highmodulus polymeric compounds used for the bi-material die attach layer510 are polymerized (cured). During step 206 of the process flow of FIG.2 and illustrated in FIG. 7, copper wire bonding is performed byattaching copper free air balls 730 to bonding pads 602 and stitch bondsof copper wire 631 to leadframe leads 322. In another exampleembodiment, flip chip connections can be used instead of wire bondingthe die bonding pads to make electrical connection to the leadframeleads. In still further example embodiments, wire bonding using wirematerials other than copper can be used.

As shown in FIG. 1, the last step of the process flow is theencapsulation of the semiconductor die 101, the bonding wires 131, andportions of the bi-material die attach layer 110 and the leadframe 120in a packaging material 140 to form a packaged integrated circuit. In anexample embodiment, the packaging material is an epoxy-based thermosetcompound suitable for transfer molding technology; in a particularexample the molding compound has about 90% inorganic fillers. Othermolding compounds suitable for semiconductor packages can be used.

FIGS. 8 to 14 illustrate views obtained by parallel lapping through avariety of adhesive layers used in varying embodiments for semiconductordevices to attach semiconductor dies to substrates. Each of FIGS. 8-14includes an assembly pad of a substrate, preferably a metallicleadframe; the pads are designated 821, 921, etc. On the assembly pad,the bi-material adhesive layers are deposited as described hereinabove.In an example process, the adhesive layers are deposited from nozzlesusing inkjet printers. Each adhesive layer includes first zones composedof a first polymeric compound and second zones composed of a secondpolymeric compound; as mentioned hereinabove, the first polymericcompound has a first modulus and the second polymeric compound has asecond modulus greater than the first modulus. The first zones aredesignated 811, 911, etc., and the second zones are designated 812, 912,etc. The zones of the second polymeric compound are configured to mirrorimage and counterbalance any bonding pads on the opposite die surface.In each device of FIGS. 8 to 14, the sum of the first zones of the firstpolymeric compound constitute a greater portion of the attach layer thanthe sum of the second zones of the second polymeric compound. However,there are other example embodiments for devices where the inversesituation prevails. In these alternative embodiments, the sum of thefirst zones of the first polymeric compound (having a first modulus)constitute a smaller portion of the attach layer than the sum of thesecond zones of the second polymeric compound (having a second modulusgreater than the first modulus).

Another embodiment of the present application is a device, which has astructure analogous to FIG. 1. The device can therefore employ the samedesignations. The device includes a semiconductor die 101 with a firstsurface 101 a and bond pads 102, and an opposite second surface 101 battached to a substrate 120 by a bi-material die attach layer 110covering the area of surface 101 b.

The bi-material die attach layer 110 includes first zones 111 composedof a first polymeric compound and adding up to a first portion of thearea of surface 101 b, and second zones 112 composed of a secondpolymeric compound and adding up to a second portion of the surfacearea. The first zones 111 and the second zones 112 are contiguous. In anexample embodiment, the polymers are epoxy-based compounds. As shown inFIGS. 8 and 9, the second portion of the surface area, formed by thesecond zones 812 and 912, respectively, can be smaller than the firstportion of the surface area, formed by the second zones 112. In otherembodiment devices, however, this ratio may be inversed.

In an example embodiment, the first polymeric compound includes metallicfillers and the second polymeric compound is essentially free ofmetallic fillers. In a particular example embodiment the metallicfillers are silver particles. Use of the silver filler in the adhesivelayer results in the electrical and thermal conductivity from die 101(see FIG. 1) to substrate 120 being enhanced. Use of the secondpolymeric compound that is free from filler lowers any risk of formingan electrically conductive dendrite from the die support pad 121 to abond pad 102. In a DC electric field, for instance, between the anodicdie support pad 121 and a nearby cathodic bond pad 102, a silver filledpolymeric compound can undergo dissolution in the presence of moistureand thereby create positive silver ions. In the presence of an adsorbedwater film acting as an electrolyte, the positive silver ions migrate inthe DC electric field from the anodic die support pad 121 (see FIG. 1)to the cathodic bond pad 102. At the bond pad, the silver ions arereduced to pure silver and can eventually form a metallic dendrite tree,which can cause an electrical short between anode (die support pad 121)and cathode (bond pad 102). Consequently, in an example embodiment, thesecond zones (112, 812 and 912 in FIGS. 1, 8 and 9, respectively)composed of metallic filler-free compound are distributed to locationsof the bi-material die attach layer (110 in FIG. 1) where a metallicdendrite from the adhesive layer to a bond pad would be likely to grow.By use of the embodiments, the growth of the metallic dendrite isreduced or prevented, reducing the risk of these electrical shorts.

For devices that include stress-sensitive circuitry and which needenhanced protection against thermos-mechanical stress, it isadvantageous to select the first polymeric compound with a first modulusand the second polymeric compound with a second modulus greater than thefirst modulus.

Another embodiment addresses the needs of devices which have circuitryrequiring an adhesive layer having different thermal conductivities,electrical conductivities, or both. Such devices have a structureanalogous to FIG. 1 and can, therefore, employ the same designations.The device includes a semiconductor die 101 with a first surface 101 aand bond pads 102, and an opposite second surface 101 b attached to asubstrate 120 by bi-material die attach layer 110 covering the area ofsurface 101 b. The bi-material die attach layer 110 includes first zones111 composed of a first polymeric compound and adding up to a firstportion of the area of surface 101 b, and second zones 112 composed of asecond polymeric compound and adding up to a second portion of thesurface area. The first zones 111 and the second zones 112 arecontiguous. In an example embodiment, the polymers are epoxy-basedcompounds. As shown in FIGS. 8 and 9, the second portion of the surfacearea, formed by the second zones 812 and 912, respectively, can besmaller than the first portion of the surface area, formed by the secondzones 112. In other embodiment devices, however, the ratio may beinversed.

In an example embodiment, the first polymeric compound includes a firstthermal conductivity and the second polymeric compound a second thermalconductivity less than the first conductivity. The different thermalconductivities may be achieved by different polymeric formulations andby different admixtures to the polymeric material. In another exampleembodiment, the first polymeric compound includes a first electricalconductivity and the second polymeric compound a second electricalconductivity less than the first electrical conductivity. The differentelectrical conductivities may be achieved by different polymericformulations and by different admixtures to the polymeric material. Inyet another example embodiment, the first polymeric compound includesboth a first thermal conductivity and a first electrical conductivity,and the second polymeric compound includes both a second thermalconductivity less than the first thermal conductivity, and a secondelectrical conductivity less than the first electrical conductivity.

For devices that include the above-mentioned conductivity-sensitivecircuitry and which in addition include stress-sensitive circuitry whereenhanced protection against thermo-mechanical stress is desirable, it isadvantageous to select the first polymeric compound not only with thefeature of a first thermal conductivity or a first electricalconductivity, but in addition with the feature of a first modulus. Thesecond polymeric compound is then selected not only with the feature ofa second thermal conductivity less than the first conductivity, or of asecond electrical conductivity less than the first electricalconductivity, but also the feature of a second modulus greater than thefirst modulus.

While various illustrative embodiments have been described, use of theexamples in this description is not intended to be construed in alimiting sense. Various modifications and combinations of theillustrative embodiments, as well as other embodiments, will be apparentupon reference to the description.

As an example, in semiconductor technology, the embodiments apply notonly to active semiconductor devices with low and high pin counts, suchas transistors and integrated circuits, but also to combinations ofactive and passive components mounted on a leadframe pad.

As another example, the embodiments apply not only to silicon-basedsemiconductor devices, but also to devices using gallium arsenide,gallium nitride, silicon germanium, and any other semiconductor materialemployed in industry. The embodiments apply to leadframes withcantilevered leads and to QFN and SON type leadframes.

As another example, the embodiments apply to laminated substrates andany other substrate or support structure that is to be bonded to anon-metallic body.

Modifications are possible in the described embodiments, and otherembodiments are possible, within the scope of the claims.

What is claimed is:
 1. A method for fabricating a device comprising:providing a first nozzle and a second nozzle, the first nozzle fordispensing a first adhesive compound having a first elasticity modulusafter curing, and the second nozzle for dispensing a second adhesivecompound having a second elasticity modulus greater than the firstelasticity modulus after curing; while the first and the second nozzleare moving relative to the surface of a substrate, dispensing first andsecond compounds for additively depositing an adhesive layer on thesurface, the first nozzle depositing first zones adding up to a firstportion of a surface area of the substrate, and the second nozzledepositing second zones adding up to a second portion of the surfacearea, the first zones and the second zones being contiguous; andattaching a second surface of a semiconductor die onto the depositedadhesive layer, the semiconductor die having an opposing first surfacewith bond pads.
 2. The method of claim 1, in which the dispensing isdone simultaneously using the first and second nozzles.
 3. The method ofclaim 1, in which the dispensing is done alternatingly using the firstand second nozzles.
 4. The method of claim 1 wherein the second portionof the surface area is smaller than the first portion of the surfacearea.
 5. The method of claim 1 further including polymerizing theadhesive layer.
 6. The method of claim 5 further including attachingcopper wire balls onto the bond pads of the semiconductor die.
 7. Themethod of claim 1 wherein providing the first and second nozzlesincludes providing parts of equipment for dispensing selected from agroup, including: inkjet printing equipment, including any ofpiezoelectric, thermal, acoustic, and electrostatic inkjet printingequipment; screen printing; flexographic printing; gravure printing; dipcoating; and spray coating.
 8. The method of claim 1 further includingmetallic fillers in the first polymeric adhesive compound, while thesecond polymeric adhesive compound is essentially free of the metallicfillers.
 9. A method for fabricating a semiconductor device comprising:providing a first nozzle and a second nozzle, the first nozzle fordispensing a first adhesive compound having metallic fillers, and thesecond nozzle for dispensing a second adhesive compound essentially freeof metallic fillers; moving the first nozzle and the second nozzlerelative to the surface of a substrate while dispensing first and secondcompounds for additively depositing an adhesive layer on the surface,the first nozzle depositing first zones adding up to a first portion ofa surface area of the surface of the substrate, and the second nozzledepositing second zones adding up to a second portion of the surfacearea, the first zones and the second zones being contiguous; andattaching a second surface of a semiconductor die onto the depositedadhesive layer, the semiconductor die having an opposing first surfacewith bond pads.
 10. The method of claim 9 in which the dispensing withthe first nozzle and the second nozzle is done simultaneously.
 11. Themethod of claim 9 in which the dispensing with the first nozzle and thesecond nozzle is done alternatingly.
 12. The method of claim 9 whereinthe second portion of the surface area is smaller than the first portionof the surface area.
 13. The method of claim 9 further includingpolymerizing the adhesive layer.
 14. The method of claim 13 furtherincluding attaching copper wire balls onto bond pads of thesemiconductor die.
 15. The method of claim 9 wherein providing the firstand second nozzles includes providing parts of dispensing equipmentselected from the group including: inkjet printing equipment, includingpiezoelectric, thermal, acoustic, and electrostatic inkjet printingequipment; screen printing; flexographic printing; gravure printing; dipcoating; and spray coating.
 16. The method of claim 9 wherein the firstadhesive compound further has a first modulus after curing and thesecond adhesive compound has a second modulus greater than the firstmodulus after curing.